In hydraulic fracturing, fracturing fluid is injected into a wellbore, penetrating a subterranean formation and forcing the fracturing fluid at pressure to crack and fracture the strata or rock. Proppant is placed in the fracturing fluid and thereby placed within the fracture to form a proppant pack to prevent the fracture from closing when pressure is released, providing improved flow of recoverable fluids, i.e., oil, gas, or water. The success of a hydraulic fracturing treatment is related to the fracture conductivity which is the ability of fluids to flow from the formation through the proppant pack. In other words, the proppant pack or matrix may have a high permeability relative to the formation for fluid to flow with low resistance to the wellbore. Permeability of the proppant matrix may be increased through distribution of proppant and non-proppant materials within the fracture to increase porosity within the fracture.
Some approaches to hydraulic fracture conductivity have constructed proppant clusters in the fracture, as opposed to constructing a continuous proppant pack. These methods may alternate the stages of proppant-laden and proppant-free fracturing fluids to create proppant clusters in the fracture and open channels between them for formation fluids to flow. Thus, the fracturing treatments result in a heterogeneous proppant placement (HPP) and a “room and pillar” configuration in the fracture, rather than a homogeneous proppant placement and consolidated proppant pack. The amount of proppant deposited in the fracture during each HPP stage is modulated by varying the fluid transport characteristics, such as viscosity and elasticity; the proppant densities, diameters, and concentrations; and the fracturing fluid injection rate.
Proppant placement techniques based on the fracture geometry have been developed for use during traditional proppant pack operations. However, proppant placement in HPP is considerably more challenging and the art is still in search of ways to improve the proppant placement techniques in HPP operations. In practice, a predetermined proppant pumping schedule was followed presuming the desired fracture geometry would result.
Prior to injection of the fracturing fluid, the proppant and other components of the fracturing fluid may be blended. The current state of technology for enabling existing blending equipment for performing HPP and slickwater fracturing operations relies on the use of automatic proppant concentration control based on proppant metering gate percentage opening in a gravity-fed system. Automatic proppant concentration control based on densitometer feedback is the most commonly used mode for proppant metering in conventional fracturing work, but cannot be used in certain applications due to densitometer response times. Additionally, current gate designs in existing blending equipment generally have irregular metering orifice geometries with respect to gate percentage opening that do not allow highly accurate and consistent proppant flow control. A means for achieving consistent, well-behaved proppant metering due to consistent, well-behaved metering orifice geometry for optimal performance is desirable.
Many proppant addition systems use one or more augers to supply proppant or a mixture of proppant and fluids, such as slickwater, gels, or hydrocarbons. In these systems, the proppant may be delivered to the fracturing fluid, pumps, or mixer from an oilfield material reservoir, commonly called a proppant hopper or receiver. The auger meters the proppant volumes and rates into a fluid stream or mixer. The auger may meter the proppant by calculating the known amount of proppant an auger may move at a given auger speed in revolutions per minute (rpm). The density of fracturing fluid including the proppant therefore may be determined, in auger systems, based on the rpm at which the auger is operating in combination with the density of the fracturing fluid determined prior to the addition of the proppant. Auger systems may require a larger area in order to accommodate an auger capable of providing a sufficient volume of proppant to the mixer or the fluid stream.
An alternative to the auger fed proppant addition systems is the use of a gravity fed proppant addition system. Gravity fed proppant addition systems may transfer proppant via gravity free fall to a mixer in order to be added to fracturing fluid. Metering the proppant volume in a gravity fed system may be calculated by determining the flow rate of the proppant through an orifice of a known size when the proppant is in gravity free fall through the orifice. Gravity fed systems may also employ the use of pressurization to aid in transferring proppants into the fluid stream or mixer. Pressurization methods in gravity fed systems may include pressurizing the proppant container subject to the gravity feed or utilizing a venturi effect where a smaller diameter pipe is connected to a larger diameter pipe to draw the proppant from the proppant container into the mixer or fluid stream. Gravity fed systems may require a smaller area, as they may not employ an auger.
Gravity fed proppant addition systems may use automatic proppant concentration control based on the orifice of known size, often called a metering gate. Blending equipment has been adapted for slickwater fracturing jobs by use of automatic proppant concentration control based on the metering gate percentage opening in the gravity fed proppant addition system. This automatic proppant concentration control may be called Auto-Concentration in Gate Percentage Mode. As previously stated, automatic proppant concentration control may be based on densitometer feedback; however densitometer feedback may not be an effective control mechanism for slickwater applications due to the inability of densitometers to differentiate between the density of low proppant concentration slurries common to slickwater fracturing and the density of the base fluid carrier itself.
Bins and hoppers with highly tapered geometries may not successfully be used with fibrous materials without the oilfield materials bridging and plugging the narrower end of the hopper. There currently exists no means in any industry of storing several thousand pounds of oilfield material in a large bin of wide cross sectional area able to transfer the material directly from such a bin through a conduit of a smaller cross sectional area.